Conventionally, there has been already known well a gas internal combustion type nailing machine (for example, see the patent reference 1) structured in the following manner: that is, a combustion chamber formed in the nailing machine includes a movable housing portion, and the movable housing portion can be opened and closed in linking with the operation of a contact member; in the closed state of the combustion chamber, combustible gas is supplied into the combustion chamber, the combustible gas and air are mixed together thereby rotating a fan, the thus mixed gas is ignited using a spark plug and is thereby combusted explosively, and the resultant high gas pressure is applied onto a striking piston to thereby drive a nail; and, in the opened state of the combustion chamber, the air is sucked and is discharged from the lower portion of a striking cylinder.
In the combustion chamber, in order to mix together the combustible gas and air quickly, the mixed gas must be stirred up at a high speed. In this case, when the flow speed of the mixed gas is high, the mixed gas is sometimes hard to be ignited even when the spark plug is operated for ignition. In view of this, it is known that there is formed a special block wall in the periphery of the spark plug (for example, see the patent reference 2).
[Patent Reference 1] JP-B-04-048589
[Patent Reference 2] JP-A-2006-224268
Here, in the above-mentioned well-known gas internal combustion type nailing machine, when driving the fan for stirring the mixed gas within the combustion chamber, in the vicinity of the central portion of the lower portion of the fan, there is formed a stagnant portion where the flow of the mixed gas is weak and the gas combustion speed is thereby retarded. This can raise a fear that the mixed gas can be combusted incompletely to degrade the combustion efficiency of the mixed gas. Also, since a guide and a seal respectively used for the operation of the movable housing of the combustion chamber are formed as separate parts which are disposed in the upper and lower portions of the movable housing, they are easy to differ in the axis thereof from each other, which raises a fear that the operation load of the movable housing of the combustion chamber can increase and the poor sealing of the movable housing can occur.
Further, since the upper and lower portions of the movable housing of the combustion chamber are different in the sealing diameter thereof from each other, such pressure within the combustion chamber as is influenced by the combustion pressure and supercharged pressure causes an unbalanced load within the combustion chamber to thereby generate an operation load there. Specifically, the load caused by the combustion pressure has an influence on the mechanical strength of the combustion chamber, whereas the load caused by the supercharged pressure can raise a fear that, after the air is supercharged, the combustion chamber is moved up to its upper position and is left unmoved there, whereby the combustion chamber cannot be opened.